Wavefront aberration is an importance parameter in depicting the properties of a small-aberration imaging optical system. Wavefront aberration of a high quality microscopic objective or a spatial telescope needs to be smaller than λ/4 PV or λ/14 RMS (where λ is an operating wavelength, and RMS is a root mean square value). Wavefront aberration of a deep UV lithographic projection lens needs to be in a few nm RMS, while that of an EUV needs to be less than 1 nm RMS, which are demanding requirements for wavefront aberration techniques.
A point diffraction interferometric wavefront aberration measuring device and a detecting method is proposed in Chinese Patent Application No. 201310126148.5 to Tang, Feng et al., two standard spherical waves are generated on an object plane of a measured optical system, the light intensity, polarization states, and optical path difference thereof being adjustable for producing high interference visibility, and the measured results being able to eliminate the system errors. Nevertheless, for one thing, the interferometer needs to generate two standard spherical waves on the object plane of the measured optical system, which increases system complexity and cost, and the alignment is extremely difficult when the standard spherical waves are generated via keyhole components. Secondly, the interferometer has low light use efficiency due to reduced light output efficiency for generation of two standard spherical waves on the object plane, and further reduced light output efficiency when re-generation of the standard spherical waves via the filtering circular hole on the image mask after imaging of the two standard spherical waves on the image plane; detection repeatability is negatively impacted due to reduced frame speed of the measuring system and increased sensitivity to vibration and other disturbance resulting from increased exposure time to obtain sufficiently clear interferograms. Thirdly, the interferometer requires that both the first output port and the second output port are situated on the object plane of the measured optical system, which increases requirement for system precision and operation complexity.